Is the doctrine of the retrobation of those who receive the mark of the beast an essential doctrine?

I felt it would be important to ask about this issue, because it relates to the Gospel. Also, I understand that many on this subreddit hold to a symbolic view of the mark of the beast, but I still thought it would be good to get someone else's opinion.

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I recently heard someone responding to the claim that you can be forgiven after taking the mark of the beast by saying:

"This is not a light issue. This is ridiculous and it is as damnable as anything Kenneth Copeland preaches"

I wasn't sure what he meant by "damnable". Perhaps he meant that this teaching could damn you if it influences you to take the mark of the beast. But he may have meant that believing the doctrine is damnable in and of itself.

I personally agree that you cannot be forgiven for taking the mark, but I'm afraid of considering it an essential doctrine because I don't want to add to the Gospel. On the other hand, if it is an essential issue, denying its importance would be damnable. What are your thoughts?